System and method for reducing the effects of clock harmonic frequencies

ABSTRACT

A system and a method are provided for reducing the effects of spurious frequencies in a wireless communications device. The system comprises a processor having a reference frequency input and a clock having an output connected to the processor input. The clock supplies a clock frequency, or reference frequency, to the processor. The reference frequency is the frequency at which the processor operates. The clock also has an input for selecting a reference frequency to provide to the processor. The system also includes a transceiver having a plurality of selectable communications passbands. If the wireless communications device is a telephone, for example, the transceiver frequency (passband) may change as a function of the mode in which the phone is operating (AMPS, PCS, GSM, CDMA, or W-CDMA). In response to changing operating modes (transceiver passband), the clock frequency is adjusted. The clock frequency is selected so that harmonic frequencies associated with the clock frequency do not substantially interfere with the transceiver passband.

RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.09/998,458, filed on Nov. 29, 2001 now U.S. Pat No. 6,999,723.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to wireless communications and, moreparticularly, to a system and method for reducing the effects of clockgenerated harmonics in the transceiver passband of a wirelesscommunications device.

2. Description of the Related Art

Processors, such as microprocessors, are an integral part of manyelectronic products, including a typical wireless communications device.Microprocessors are typically used to execute instruction sets and togenerally control the operation of devices. A clock is used with thetypical microprocessor to provide a frequency at which themicroprocessor operates. A crystal is a conventional and inexpensive wayof supplying a microprocessor reference frequency. As is well known,spurious frequencies such as harmonics, are generated by the clock,along with the desired clock frequency (reference frequency). Thesespurious frequencies, as well as the crystal resonant frequency, “leak”through to other circuits on the device printed circuit board throughconductance on the power lines or grounds, and through radiation. Theseclock frequencies can interfere with the operation of a device byunintentionally mixing with other frequencies that are generated on theboard.

A wireless communication device transceiver circuit is especiallysusceptible to interference from spurious signals. In some instances,the spurious signals have frequencies equal to the carrier frequency ofthe wireless communications device. These spurious signals may provideundesirable noise and degrade performance of the device. For example,noise in the passband of a receiver can degrade sensitivity, while noisein a transmitter passband can violate system specifications, or raisethe general noise floor of the system in which the wireless device isoperating.

To solve spurious signal and interference problems, it is typicallynecessary to add filtering circuits or shielding to the circuit board.However, this solution takes time and adds to the total parts count. Insome instances, the size and position of the board may limit thefiltering that can be provided. It is known to change the clockfrequency to avoid harmonic interference with the local oscillator orother transceiver circuitry. However, it is not desirable to reselectwireless device crystals to accommodate printed circuit board filteringissues. In wireless devices that operate with more than one transceiverfrequency, it may be difficult to find a single clock frequency thatsupports the operation of the digital circuitry, and yet avoidsinterfering with at least one of the transceiver frequencies.

It would be advantageous if the undesirable effects of spuriousfrequencies, such as those harmonically related to a clock, could beeliminated from or substantially reduced in the passband of atransceiver without special filtering.

It would be advantageous if a wireless communications device could beoperated at a plurality of transceiver frequencies while reducing thenoise effects of spurious signals related to clock generated harmonics.

SUMMARY OF THE INVENTION

The present invention was created to address the problem of reducing theeffects of clock-generated harmonics in the transceiver passband of awireless communications device. The invention recognizes that theharmonics generated by a clock can leak through to the transceiver andcause sensitivity, noise, and system specifications problems in thewireless device. The invention also recognizes that the avoidance ofharmonics by the use of filtering circuits or shielding, or by changingcrystals is not always desirable or effective. The invention addressesthe above-mentioned problem by selectively adjusting clock frequenciesso that the harmonics associated with the selected clock frequency donot substantially interfere with the operating passband of thetransceiver.

Accordingly, a system is provided for reducing the effects of spuriousfrequencies that substantially interfere with the performance of awireless communications device. The system comprises a processor havinga reference frequency input and a clock having an output connected tothe processor input to supply the clock frequency, or referencefrequency. The reference frequency is the frequency at which theprocessor operates. The clock has an input for selecting a referencefrequency to provide to the processor.

The system also includes a transceiver supplying a plurality ofselectable communications passbands. If the wireless communicationsdevice is a telephone for example, the transceiver frequency (passband)changes as a function of the mode in which the phone is operating(advanced mobile phone service (AMPS), personal communication services(PCS), global system for mobile communications (GSM), code divisionmultiple access (CDMA), or wideband CDMA (W-CDMA)). In response tochanging operating modes, the clock adjusts the clock frequency. Theclock frequency is selected so that harmonic frequencies associated withthe clock frequency do not substantially interfere with the transceiverpassband.

Additional details of the above-described system and a method forreducing the effects of spurious frequencies in a wireless device arepresented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram depicting the system for reducingthe effects of spurious frequencies in a wireless communications devicein accordance with the present invention.

FIG. 2 is a flowchart illustrating the method for reducing the effectsof spurious frequencies in a wireless communications device inaccordance with the present invention.

FIG. 3 is a flowchart illustrating the method for avoiding spuriousfrequencies in the transceiver passband of a wireless communicationsdevice in accordance with the present invention.

FIG. 4 is a flowchart illustrating the method for reducing the effectsof clock harmonics in the passband of a wireless communications devicein accordance with the present invention.

FIG. 5 is a flowchart illustrating another aspect of the method forreducing the effects of clock harmonics in the passband of a wirelesscommunications device in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic block diagram depicting the system 100 forreducing the effects of spurious frequencies in a wirelesscommunications device 102 in accordance with the present invention. Thedevice 102 includes a microprocessor 104 with an input on line 106 toaccept a reference clock frequency. A clock 108 has an output on line106 to supply a clock (reference) frequency to the microprocessor 104.This clock frequency is often the fundamental frequency of a crystalsource. The clock 108 is capable of providing a plurality ofpredetermined clock frequencies, as described below.

It will be appreciated that other types of processors and designs may besubstituted for the microprocessor and clock. For example, a processorand a clock may be provided on an application specific integratedcircuit as a single component. Also, other types of processors, such asgate arrays or other programmable logic devices may by used.

A transceiver 110 has a port to transmit and receive a carrier frequencyvia an airlink interface. The airlink interface is represented byantenna 112. The transceiver 110 may include de/modulating circuits,power amplifiers, receivers, frequency sources, and other components(not shown), but well known in the art. The transceiver is typically amultiband transceiver having an input on line 114 for accepting commandsselecting one of a plurality of communication passbands. Alternately butnot shown, the device 102 includes a plurality of transceivers, eachtuned to operate in a different passband.

Harmonics are generated by the clock 108, along with the desired clockfrequency needed to operate the microprocessor 104. These harmonics canappear as spurious frequencies that can radiate or conduct into thetransceiver 110, interfering with the carrier frequencies. The radiationor conduction of these spurious frequencies are represented by thedashed line with the reference designator 116. Although the harmonicscan be filtered or shielded from the transceiver 110, this problem isoften more directly solved by selecting a crystal frequency withharmonics that avoid the transceiver passband. However, selecting acrystal frequency that generates harmonics outside the transceiverpassband can become difficult, or even impossible, if the device 102 hasmultiple transceivers or if the transceiver 110 is expected to operateat more than one carrier frequency (as shown).

To this end, the microprocessor 104 operates at more than one clock(reference) frequency. The clock 108 has an input on line 118 to acceptselections for reference frequencies, and supplies clock frequencies inresponse to the selections on line 118. In some aspects of the system100, the clock 108 can be a bank of selectable crystals or a frequencysynthesis system.

The following example illustrates the operation of the present system100. When the wireless device 102 is operating in a first mode, such ascode division multiple access (CDMA), the clock 108 provides a firstreference frequency to the microprocessor 104. In turn, the transceiver110 operates in the CDMA passband of approximately 1900 megahertz (MHz).The CDMA passband is selected in response to commands on line 114. Whenthe wireless device 102 switches to a second mode, such as the AMPS(analog) mode, in response to commands on line 114, the transceiver 110switches passband frequencies. The AMPS center frequency isapproximately 900 MHz.

If the first reference frequency generated by clock 108 has anassociated harmonic that occurs at 900 MHz, the transceiver signals maybe degraded when the transceiver 110 switches to the AMPS mode. To avoidsuch interference, a second reference frequency is selected using theclock 108 input on line 118. In response, the clock 108 provides thesecond reference frequency to the microprocessor 104 on line 106. Thesecond reference frequency is predetermined so that the harmonicsassociated with it do not interfere with the second mode (AMPS)passband.

More specifically, if the first reference frequency is 19.2 MHz, the46th harmonic is 883.2 MHz. This is a frequency that does not interferewith the CDMA passband centered around 1900 MHz. However, the 46thharmonic does interfere with the transceiver 110 when switched into theAMPS mode at approximately 900 MHz. To solve the interference problem,the second reference frequency is selected to be 26.24 MHz. The 46thharmonic at 1207 MHz is far removed from the AMPS passband.

FIG. 2 is a flowchart illustrating the method for reducing the effectsof spurious frequencies in a wireless communications device inaccordance with the present invention. Although the method (and theother methods described below) is depicted as a sequence of numberedsteps for clarity, no order should be inferred from the numbering unlessexplicitly stated. It should be understood that some of these steps maybe skipped, performed in parallel, or performed without the requirementof maintaining a strict order of sequence. The method starts at Step200. Step 202 provides a plurality of selectable passband ranges for thewireless communications device. Step 204 selects one of the passbandfrequency ranges. Step 206 determines a clock frequency that produces nosubstantial spurious signals (spurs) in the selected passband frequencyrange. Step 208 adjusts a clock to generate a clock signal at the clockfrequency. Step 210 drives a processor with the clock signal. In oneaspect of the invention, selecting one of the passband frequency rangesin Step 204 includes providing a cellular passband frequency range and aPCS passband frequency range.

FIG. 3 is a flowchart illustrating the method for avoiding spuriousfrequencies in the transceiver passband of a wireless communicationsdevice in accordance with the present invention. The method starts atStep 300. Step 302 generates a clock frequency with harmonics at a firstfrequency. Step 304 generates a transceiver carrier at a secondfrequency. Step 306 selects the clock frequency so that the firstfrequency does not equal the second frequency.

In some aspects of the method, generating a transceiver carrier at asecond frequency in Step 304 includes generating a transceiver carrierwith a center frequency of approximately 900 MHz. A further step, Step301, initially generates a clock frequency at 19.2 megahertz (MHz) witha 46th harmonic at 883.2 MHz. Then, selecting the clock frequency sothat the first frequency does not equal the second frequency in Step 306includes increasing the clock frequency from 19.2 MHz to 26.24 MHz.

FIG. 4 is a flowchart illustrating the method for reducing the effectsof clock harmonics in the passband of a wireless communications devicein accordance with the present invention. The method starts at Step 400.Step 402 generates a clock frequency with harmonics at a firstfrequency. Step 404 generates a transceiver carrier at a secondfrequency. Step 406 changes the clock frequency so that the firstfrequency does not equal the second frequency.

FIG. 5 is a flowchart illustrating another aspect of the method forreducing the effects of clock harmonics in the passband of a wirelesscommunications device in accordance with the present invention. Themethod starts at Step 500. Step 502 generates a clock frequency withharmonics at a first frequency. Step 504 generates a transceiver carrierat a second frequency not equal to the first frequency. Step 506 changesthe transceiver carrier to the first frequency. Step 508 changes theclock frequency to a clock frequency with harmonics at a thirdfrequency, not equal to the first frequency.

A system and a method are provided for reducing the effects of spuriousfrequencies in the transceiver passband of a wireless communicationsdevice. Examples of the invention have featured specific referencefrequencies, passband frequencies, and modes of operation, however, itshould be understood that the present invention is not limited to anyparticular frequencies or modes of operation. Examples have also beengiven of a wireless telephone device, however, it should be understoodthat the invention is applicable to a broader field of wirelessoperations. Other variations and embodiments of the invention will occurto those skilled in the art.

1. A method for reducing effects of spurious frequencies in a wirelesscommunications device, the method comprising: powering up the wirelesscommunications device in a first mode of operation comprising one of ananalog mode and a digital mode, the first mode of operationcorresponding to a first passband frequency range of a plurality ofselectable passband frequency ranges; operating a processor in a defaultprocessor clock frequency of a plurality of processor clock frequencies,the default processor clock frequency corresponding to a second mode ofoperation comprising the other of the analog mode and the digital mode;determining that the default processor clock frequency produces spurioussignals in the first passband frequency range; switching to a firstprocessor clock frequency of the plurality of processor clockfrequencies that produces no substantial spurious signals in the firstmode of operation; and maintaining the first processor clock frequencyas song as the wireless communications device operates in the first modeof operation.
 2. The method of claim 1, wherein the analog modecomprises advanced mobile phone service (AMPS).
 3. The method of claim1, wherein the digital mode comprises a code division multiple access(CDMA) mode.
 4. The method of claim 1, wherein the first mode ofoperation is an analog mode; and wherein the default processor clockfrequency corresponds to a digital mode.
 5. The method of claim 1,wherein the digital mode comprises one of a global system for mobilecommunications (GSM) mode, and a code division multiple access (CDMA)mode.
 6. A wireless communication device comprising: a microprocessorhaving a reference frequency input, a clock selection output, and acommand selection output, the microprocessor for selecting one of atleast two operation modes comprising a digital mode and an analog mode,and for outputting the selected operation mode on the command selectionoutput; a clock having a clock output connected to the referencefrequency input of the microprocessor, and an input connected to theclock selection output of the microprocessor, the clock outputting adefault frequency clock corresponding to a first operation mode of theat least two operation modes upon a start up condition, the firstfrequency clock producing a first set of harmonic frequencies; amultiband transceiver for transceiving a plurality of selectablecommunication passbands, the multiband transceiver having at atransceiver input connected to the command selection output of themicroprocessor, the plurality of selectable communication passbandscomprising: a first communication passband corresponding to the firstoperation mode; and a second communication passband corresponding to asecond operation mode, wherein the first set of harmonic frequenciesinterferes with the second communication passband; wherein themicroprocessor selects a second frequency clock having a second set ofharmonic frequencies that do not interfere with the second communicationpassband when the second communication passband is active upon the startup condition, and maintains the second frequency clock on the referencefrequency input as long as the second communication passband isselected.
 7. The system of claim 6, wherein the analog mode comprises:advanced mobile phone service (AMPS).
 8. The system of claim 6, whereinthe digital mode comprises one of a global system for mobilecommunications (GSM) mode, and a code division multiple access (CDMA)mode.
 9. The system of claim 6, wherein the clock is connected to acrystal source, and the default frequency clock is a fundamentalfrequency of the crystal source.
 10. The system of claim 6, wherein thesecond operation mode is an analog mode; and wherein the defaultfrequency clock corresponds to a digital mode.